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I've run into a very odd problem. I've created the attached circuit, designed to cut power supply when a magnet is brought near the device. It works perfectly with the reed switch disconnected from the circuit. However, when everything is powered up, the reed switch doesn't react to the magnet at all unless I put an oscilloscope probe on pin 2 (NC) of the reed switch, at which point everything works as expected. Datasheet for the reed switch.

schematic

SamGibson
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Alex
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  • The pulldown is disabled when regulator is enabled so it does not pull down. EN pin must be brought down externally with at least 0.7uA of sinking current. – Justme Jul 28 '20 at 08:12
  • The scope probe acts as a (10 Megohm) pulldown. –  Jul 28 '20 at 09:14

2 Answers2

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You're reed switch will pull your enable pin high when closed, but will float the pin when open.

When you connect the scope probe, this acts like a large (1MOhm) pull-down resistor which will set the enable pin low when the reed switch is open. This is important as the internal pull-down resistor on the pin is disconnected when the device is enabled, so cannot be relied upon.

Internal circuit

A simple fix is to connect a pull-down resistor, say 10k or 100k, between EN and GND. This will pull the pin low when the reed switch is open and ensure the device is shut down.

Tom Carpenter
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The enable pin must not be left disconnected, it has internal circuitry connected to it to for example provide hysteresis, so if it is high it might keep itself high. So some current must be pulled out of the pin to make it low so the regulator gets disabled. Try connecting a 100k resistor from EN to ground or just grounding the NO pin.

Justme
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